Control of protein synthesis (translation) is vital for cell proliferation, differentiation or death. We found that human eukaryotic translation initiation factors elF1 A and elF5 compete for the same surface of the G-protein elF5B, and that elF5B may be transferred from elF5 to elF1 A, which would allow for a switch-type control of the corresponding step. elFSB and elF1 A are the only two translation initiation factors conserved among all kingdoms. elF5 is the GTPase-activating protein (GAP) for the other G-protein, elF2. GTP hydrolysis by elF2 upon start site recognition is required for ribosomal subunit joining, which is promoted by elFSB. Thus, the elF5B:elF5/elF1A interactions may coordinate the two GTP hydrolysis checkpoints in translation initiation. The competition between elF5 and elF1 A for elFSB allows to study both the spatial and the temporal aspects of the regulation of translation initiation. Furthermore, the potential of using inhibition of the interactions of elFSB for anti-cancer therapy remains to be explored. The research will pursue the following specific aims: 1. Determine the structure of the complex of human elF5B-CTD with the elF5 C-terminal tail by Nuclear Magnetic Resonance (NMR);establish the basis for binding affinity and specificity. 2. Identify the roles of the elF5B:elF5/elF1 A interactions in translation initiation, what types of mRNAs are affected and at which stages of initiation. Test whether and when elFSB is transferred from elF5 to elF1 A. 3. Screen for small-molecule inhibitors of the interactions of elFSB with elF1 A/elF5. Explore the potential of using these inhibitors as drug candidates for cancer therapy. The results anticipated from this project will answer important questions in translation initiation. They will also allow me to expand my research in new directions, and serve as a foundation for building a successful career in cancer research. Relevance to cancer research: Actively dividing cells, such as cancer cells, require increased rates of protein synthesis. The potential of the regulation of protein synthesis for cancer therapy has only recently started to attract attention. The work outlined in this proposal will help establish new targets and explore new approaches in using modulation of the rates of protein synthesis for cancer therapy, such as blocking the increase in the supply of amino acids in response to the high demands in rapidly dividing cells.